Network Working Group J. Arkko
Internet-Draft A. Keranen
Intended status: Informational Ericsson
Expires: January 27, 2012 July 26, 2011
CoAP Security Architecturedraft-arkko-core-security-arch-00
Abstract
Constrained Application Protocol (CoAP) is a light-weight protocol
designed to be used in machine-to-machine applications. This memo
describes challenges associated with securing CoAP and proposes a new
security model that the authors believe is suitable for these
environments. The model requires minimal amount of configuration,
but still provides strong security and is a natural fit with the
typical communication practices smart object networking environments.
This memo also proposes JSON payload format extensions to support the
architecture.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 27, 2012.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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Internet-Draft CoAP Security July 20111. Introduction
Constrained Application Protocol (CoAP) [I-D.ietf-core-coap] is a
light-weight protocol designed to be used in machine-to-machine
applications such as smart energy and building automation.
This memo describes implementation and operational challenges
associated with securing CoAP in these environments (Section 3),
reviews related work in solving these challenges (Section 2), and
proposes a security model (Section 4) that the authors believe is
suitable for many machine-to-machine application environments. The
model requires minimal amount of configuration, but still provides
strong security and is a natural fit with the typical communication
practices smart object networking environments. Finally, this memo
proposes some protocol and payload format extensions to support the
architecture (Section 5). Section 6 provides a summary of the
approach.
2. Related Work
CoAP base specification [I-D.ietf-core-coap] outlines how to use DTLS
[RFC5238] and IPsec [RFC4306] for securing the protocol. DTLS can be
applied with group keys, pairwise shared keys, or with certificates.
The security model in all cases is mutual authentication, so while
there is some commonality to HTTP in verifying the server identity,
in practice the models are quite different. The specification says
little about how DTLS keys are managed.
The IPsec mode is described with regards to the protocol
requirements, noting that small implementations of IKEv2 exist
[I-D.kivinen-ipsecme-ikev2-minimal]. However, the specification is
silent on policy and other aspects that are normally necessary in
order to implement interoperable use of IPsec in any environment
[RFC5406].
[I-D.garcia-core-security] discusses the overall security problem for
Internet of Things devices. It also discusses various solutions,
including IKEv2/IPsec [RFC4306], TLS/SSL [RFC5246], DTLS [RFC5238],
HIP [RFC5201] [I-D.ietf-hip-rfc5201-bis] [I-D.moskowitz-hip-rg-dex],
PANA [RFC5191], and EAP [RFC3748]. The draft also discusses various
operational scenarios, bootstrapping mechanisms, and challenges
associated with implementing secity mechanisms in these environments.
[I-D.iab-smart-object-workshop] gives an overview of the security
discussions at the March 2011 IAB workshop on smart objects. The
workshop recommended that additional work is needed in developing
suitable credential management mechanisms (perhaps something similar
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to the Bluetooth pairing mechanism), understanding the
implementability of standard security mechanisms in small devices
(see, for instance, [I-D.kivinen-ipsecme-ikev2-minimal]), and
additional research in the area of lightweight cryptographic
primitives.
[I-D.sarikaya-core-sbootstrapping] discusses the bootstrapping
problem with low-powered nodes, and argues that this problem should
be solved at a general level and not left to link layer specific
mechanisms. The draft looks at EAP [RFC3748], PANA [RFC5191], HIP
Diet Exchange (HIP-DEX) [I-D.moskowitz-hip-rg-dex], and 802.1X
[IEEE.802-1X.2010] as potential solutions for bootstrapping.
[I-D.moskowitz-hip-rg-dex] defines a light-weight version of the HIP
protocol for low-power nodes. This version uses a fixed set of
algorithms, elliptic curve cryptography, and eliminates hash
functions. The protocol still operates based on host identities, and
runs end-to-end between hosts, protecting IP layer communications.
[RFC6078] describes an extension of HIP that can be used to send
upper layer protocol messages without running the usual HIP base
exchange at all.
[I-D.daniel-6lowpan-security-analysis] makes a comprehensive analysis
of security issues related to 6LOWPAN networks, but its findings also
apply more generally for all low-powered networks. Some of the
issues this document discusses include the need to minimize the
number of transmitted bits and simplify implementations, threats in
the smart object networking environments, and the suitability of
6LOWPAN security mechanisms, IPsec, and key management protocols for
implementation in these environments.
Cryptographically Generated Addresses (CGAs) [RFC3972] and Host
Identity Protocol (HIP) [RFC5201] have employed similar ideas as
those proposed in this memo, though with slightly different purpose
in mind, and at a different protocol layer. Similarly, PGP [RFC4880]
and other similar tools have popularized the concept of exchanging
key fingerprint values off-line. This is very similar to what is
proposed in this memo.
[I-D.rescorla-jsms], [I-D.jones-json-web-signature], and
[I-D.jones-json-web-token] propose JSON extensions similar to those
discussed in this memo, though constructed for other purposes.
Further work is needed to analyze if these proposals could be used as
a basis for smart object security communication security as well.
Obviously, general-purpose JSON signature mechanisms should be used
if they exist, even if some additional data elements might have to be
defined to carry all the information that this memo requires.
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Internet-Draft CoAP Security July 20113. Challenges
This section discusses three challenges: implementation difficulties,
practical provisioning problems, and layering and communication
models.
The most often discussed issues in the security for the Internet of
Things relates to implementation difficulties. The desire to build
small, battery-operated, and inexpensive devices drives the creation
of devices with a limited protocol and application suite. Some of
the typical limitations include running CoAP instead of HTTP, limited
support for security mechanisms, limited processing power for long
key lengths, sleep schedule that does not allow communication at all
times, and so on. In addition, the devices typically have very
limited support for configuration, making it hard to set up secrets
and trust anchors.
The implementation difficulties are important, but they should not be
overemphasized. It is important to select the right security
mechanisms and avoid duplicated or unnecessary functionality. But at
the end of the day, if strong cryptographic security is needed, the
implementations have to support that. Also, the use of the most
lightweight algorithms and cryptographic primitives is useful, but
should not be the only consideration in the design. Interoperability
is also important, and often other parts of the system, such as key
management protocols or certificate formats are heavier to implement
than the algorithms themselves.
The second challenge relates to practical provisioning problems.
These are perhaps the most fundamental and difficult issue, and
unfortunately often neglected in the design. There are several
problems in the provisioning and management of smart object networks:
o Small devices have no natural user interface for configuration
that would be required for the installation of shared secrets and
other security-related parameters. Typically, there is no
keyboard, no display, and there may not even be buttons to press.
Some devices may only have one interface, the interface to the
network.
o Manual configuration is rarely, if at all, possible, as the
necessary skills are missing in typical installation environments
(such as in family homes).
o There may be a large number of devices. Configuration tasks that
may be acceptable when performed for one device may become
unacceptable with dozens or hundreds of devices.
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o Network configurations evolve over the lifetime of the devices, as
additional devices are introduced or addresses change. Various
central nodes may also receive more frequent updates than
individual devices such as sensors embedded in building materials.
Finally, layering and communication models present difficulties for
straightforward use of the most obvious security mechanisms. Smart
object networks typically pass information through multiple
participating nodes [I-D.arkko-core-sleepy-sensors] and end-to-end
security for IP or transport layers may not fit such communication
models very well. The primary reasons for needing middleboxes
relates to the need to accommodate for sleeping nodes as well to
enable the implementation of nodes that store or aggregate
information.
4. Proposed Architecture
The proposed security architecture describes both a deployment model
for provisioning as well as a technical model for networks and
protocols.
The basis of the architecture are self-generated secure identities,
similar to Cryptographically Generated Addresses (CGAs) [RFC3972] or
Host Identity Tags (HITs) [RFC5201]. That is, we assume the
following holds:
I = h(P|O)
where I is the secure identity of the device, h is a hash function, P
is the public key from a key pair generated by the device, and O is
optional other information.
4.1. Provisioning
As provisioning security credentials, shared secrets, and policy
information is difficult, the provisioning model is based only on the
secure identities. A typical network installation involves physical
placement of a number of devices while noting the identities of these
devices. This list of short identifiers can then be fed to a central
server as a list of authorized devices. Secure communications can
then commence with the devices, at least as far as information from
from the devices to the server is concerned, which is what is needed
for sensor networks. Actuator networks and server-to-device
communication is covered in Section 4.4.
Where necessary, the information collected at installation time may
also include other parameters relevant to the application, such as
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the location or purpose of the devices. This would enable the server
to know, for instance, that a particular device is the temperature
sensor for the kitchen.
Collecting the identity information at installation time can be
arranged in a number of ways. The authors have employed a simple but
not completely secure method where the last few digits of the
identity are printed on a tiny device just a few millimeters across.
Alternatively, the packaging for the device may include the full
identity (typically 32 hex digits), retrieved from the device at
manufacturing time. This identity can be read, for instance, by a
bar code reader carried by the installation personnel. (Note that
the identities are not secret, the security of the system is not
dependent on the identity information leaking to others. The real
owner of an identity can always prove its ownership with the private
key which never leaves the device.) Finally, the device may use its
wired network interface or proximity-based communications, such as
Near-Field Communications (NFC) or Radio-Frequency Identity tags
(RFIDs). Such interfaces allow secure communication of the device
identity to an information gathering device at installation time.
No matter what the method of information collection is, this
provisioning model minimizes the effort required to set up the
security. Each devices generates its own identity in a random,
secure key generation process. The identities are self-securing in
the sense that if you know the identity of the peer you want to
communicate with, messages from the peer can be signed by the peer's
private key and it is trivial to verify that the message came from
the expected peer. There is no need to configure an identity and
certificate of that identity separately. There is no need to
configure a group secret or a shared secret. There is no need to
configure a trust anchor. In addition, the identities are typically
collected anyway for application purposes (such as identifying which
sensor is in which room). Under most circumstances there is actually
no additional configuration effort from provisioning security.
4.2. Device Groups
In some deployment cases it is also possible to configure the
identity of an entire group of devices, rather than registering the
individual devices. For instance, many installations employ a kit of
devices bought from the same manufacturer in one package. It is easy
to provide an identity for such a set of devices as follows:
Idev = h(Pdev|Potherdev1|Potherdev2|...|Potherdevn)
Igrp = h(Pdev1|Pdev2|...|Pdevm)
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where Idev is the identity of an individual device, Pdev is the
public key of that device, and Potherdevi are the public keys of
other devices in the group. Now, we can define the secure identity
of the group (Igrp) as a hash of all the public keys of the devices
in the group (Pdevi).
The installation personnel can scan the identity of the group from
the box that the kit came in, and this identity can be stored in a
server that is expected to receive information from the nodes. Later
when the individual devices contact this server, they will be able to
show that they are part of the group, as they can reveal their own
public key and the public keys of the other devices. Devices that do
not belong to the kit can not claim to be in the group, because the
group identity would change if any new keys were added to Igrp.
4.3. Protocol Architecture
As noted above, the starting point of the architecture is that nodes
self-generate secure identities which are then communicated out-of-
band to the peers that need to know what devices to trust. To
support this model in a protocol architecture, we also need to use
these secure identities to implement secure messaging between the
peers, explain how the system can respond to different types of
attacks such as replay attempts, and decide at what protocol layer
and endpoints the architecture should use.
Securing the messages is straightforward. A node with identity I
should sign each message it sends with the private key associated
with the identity I. This allows the recipient to verify that the
message was constructed by the sender. This is similar to what
Secure Neighbor Discovery (SEND) does with its RSA Signature Option
[RFC3971].
However, this simple model needs some enhancements to be able to
withstand denial-of-service and replay attacks. As we expect
connectivity in smart object networks to be intermittent, traditional
active methods such as nonce exchanges are not suitable. Instead, an
optional timestamp-based approach SHOULD be used in addition to the
basic signatures. This approach is similar to the one used to secure
unsolicited SEND messages. Nodes that implement the timestamp
approach need to have a real-time clock or they need to synchronize
to one using a network time protocol [RFC5905]. Additionally, nodes
that have persistent memory, SHOULD implement a monotonically
increasing sequence number. Message recipients SHOULD silently
ignore messages when they see a timestamp value that is out of range
from the current time plus or minus a small time drift factor.
Similarly, recipients that have seen multiple messages from the same
sender SHOULD silently ignore messages that do not have a sequence
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number greater than the one they have seen last.
These exchanges are basic cryptographic protocol tools, and have been
used in different layers of the IP protocol stack for different
purposes. For instance, HIP in its opportunistic mode could be used
to implement largely the same functionality at the IP layer.
However, it is our belief that the right layer for this solution is
at the application layer. More specifically, in the data formats
transported in the payload part of CoAP. This approach provides the
following benefits:
o Ability for intermediaries to act as caches to support different
sleep schedules, without the security model being impacted.
o Ability for intermediaries to be built to perform aggregation,
filtering, storage and other actions, again without impacting the
security of the data being transmitted or stored.
o Ability to operate in the presence of traditional middleboxes,
such as a protocol translators or even NATs (not that we recommend
their use in these environments).
Note that there is no requirement that the secure identities be
associated with IP addresses. They can certainly be used as input
material for constructing addresses for stateless address
autoconfiguration [RFC4862], but this is not required.
4.4. Actuator Networking
The above architecture is a perfect fit for sensor networks where
information flows from large number of devices to small number of
servers. But it is not sufficient alone for other types of
applications. For instance, in actuator applications a large number
of devices need to take commands from somewhere else. In such
applications it is necessary to secure that the commands come from an
authorized source.
This can be supported, with some additional provisioning effort and
optional pairing protocols. The basic provisioning approach is as
described in Section 4.1, but in addition there must be something
that informs the devices of the identity of the trusted server(s).
There are multiple ways to provide this information. One simple
approach is to feed the identities of the trusted server(s) to
devices at installation time. This requires either a separate user
interface, local connection (such as USB), or using the network
interface of the device for configuration. In any case, as with
sensor networks the amount of configuration information is minimized:
just one short identity value needs to be fed in. Not both an
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identity and a certificate. Not shared secrets that must be kept
confidential. An even simpler provisioning approach is that the
devices in the device group discussed in Section 4.2 trust each
other. Then no configuration is needed at installation time.
When both peers know the expected cryptographic identity of the other
peer off-line, secure communications can commence.
Alternatively, various pairing schemes can be employed. Note that
these schemes can benefit from the already secure identifiers on the
device side. For instance, the server can send a pairing message to
each device after their initial power-on and before they have been
paired with anyone, encrypted with the public key of the device. As
with all pairing schemes that do not employ a shared secret or the
secure identity of both parties, there are some remaining
vulnerabilities that may or may not be acceptable for the application
in question.
In any case, the secure identities help again in ensuring that the
operations are as simple as possible. Only identities need to be
communicated to the devices, not certificates, not shared secrets or
IPsec policy rules.
5. Proposed Protocol Extensions
The concrete implementation of the proposed architecture involves a
specification for the identity format and generation, and a
specification of the data format necessary to carry the signature,
public key, timestamp, and sequence number data objects.
The data format part of this specification could be implemented in
various ways, as S/MIME data [RFC3851], XML signatures [RFC3275], or
as additional data in JSON [I-D.jennings-senml] [RFC4627]. We have
chosen to use the JSON format in this memo.
5.1. Identity Format
The format of identifiers in binary representation is 128-bit
identifiers. These identifiers have no association with any existing
number space managed by IANA. In particular, they are not part of
the IPv6 address space; they exist at application layer.
The identifiers can be represented in textual form as Universal
Resource Names (URNs), with the format "device:cgi-HEX" where
"device" is the designated new URN type, "cgi" is a subtype that
stands for cryptographically generated identifiers, and HEX is an
exactly 32 characters long string of hex digits.
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While not at the right layer from the point of view of our
architecture, these identities could also be used in the Authority
Name part of CoAP DTLS (Section 10 of [I-D.ietf-core-coap]), IKE
or other lower-level protocols.
5.2. Identity Generation
The process of generating a new identity takes two input values: the
public key of the identity owner as a DER-encoded ASN.1 structure of
the type SubjectPublicKeyInfo, and optional other parameters.
An identity and associated Identity Parameters Block (defined further
below) SHOULD be generated as follows:
1. Generate a modifier, a random or pseudo-random 128-bit value.
2. Concatenate from left to right the modifier value, the encoded
public key, and any optional other parameters. Execute the SHA-
256 algorithm [FIPS.180-3.2008] on the concatenation. Take the
128 leftmost bits of the SHA-256 hash value. The result is the
identity.
3. Form an Identity Parameters Block data structure by concatenating
from left to right the modifier value, the encoded public key,
and any optional other parameters.
The output of the address generation algorithm is a new identity and
a new Identity Parameters Block data structure. The latter data
structure has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| |
+ Modifier (16 octets) +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Public Key (variable length) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Optional other parameters (variable length) ~
| |
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Public Key field MUST be formatted as a DER-encoded
[CCITT.X690.2002] ASN.1 structure of the type SubjectPublicKeyInfo,
defined in the Internet X.509 certificate profile [RFC3280]. RSA
public/private key pair SHOULD be used. When RSA is used, the
algorithm identifier MUST be rsaEncryption, which is
1.2.840.113549.1.1.1, and the RSA public key MUST be formatted by
using the RSAPublicKey type as specified in Section 2.3.1 of RFC 3279
[RFC3279].
The other parameters is a sequence of extension blocks with the
following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extension Type | Extension Data Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Extension Data ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Where
Extension Type
16-bit identifier of the type of the Extension Field. Identifier
for the one currently defined extension is defined in
Section 5.2.1, and some reserved values and values for testing use
are given in Section 8. The summary of the defined values is as
follows:
Value Name
-------------------------------------------------
0x0000 Reserved (Section 8)
0x0001 Identifier_Group (Section 5.2.1)
0xFFFD Exp_FFFD (Section 8)
0xFFFE Exp_FFFE (Section 8)
0xFFFF Exp_FFFF (Section 8)
Extension Data Length
16-bit unsigned integer. Length of the Extension Data field of
this option, in octets.
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Extension Data
Variable-length field. Extension-Type-specific data.
5.2.1. Identifier Groups
This extension has the Extension Type 0x0001 (Identifier_Group). The
purpose of the extension is to carry the public keys of other devices
in a group of devices. As discussed in Section 4.2, this can be used
to show membership of a group and ease the provisioning process.
The extension data should consist of a 16-bit length field that
expresses the number of public keys that follow, followed by each
public key, encoded as described in Section 5.2.
5.3. JSON Identity
Messages that employ secure identities and carry JSON [RFC4627]
payloads need to carry information about the identity of the device
that ultimately provided the payload. This information is necessary
to understand the source of the information, and is also necessary to
verify a cryptographic signature attached to the payload. However,
the mechanisms for transporting information about the identity and
making a signature are kept separate.
An identity is represented by a two-field object in JSON, for
instance:
{ "id": "device:cgi-27611bc81020716627ff0000cfaa1234",
"ipb": "4e26b808cd05d4e26b80912ae3e26b809143fe4e26b4GFTR35f8266" }
The "id" field MUST be included, and an additional "ipb" field for
the Identity Parameters Block MAY be included. To save
communications bandwidth, the optional field MAY be omitted even when
the sender has the information. However, the "ipb" field SHOULD
appear frequently enough in messages that recipients have likely
cached it.
5.3.1. The id Field
This field MUST contain an identity string in the format defined in
Section 5.1.
5.3.2. The ipb Field
This field MUST contain the BASE64-encoded Identity Parameters Block
associated with the same identity as given in the "id" field.
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Messages that employ secure identities and carry JSON [RFC4627]
payloads need to carry enough information to prove that the message
came from the right source. The JSON Signature Envelope is a JSON
object that carries a signature. Together with the JSON identity
fields it becomes possible for the recipients to verify the
signature. This object can be used to implement secure communication
for devices that have the secure identifiers described above and that
use JSON to transport information. Other signature envelope formats
are needed for other payload formats, but the authors believe that
the JSON format is widely applicable to smart objects.
Note that multiple competing ways to represent signature envelopes in
JSON are under development [I-D.rescorla-jsms],
[I-D.jones-json-web-signature], and [I-D.jones-json-web-token]. The
exact choice of encoding remains to be determined; this memo provides
its own signature envelope format only for completeness.
Every secure message MUST carry a JSON envelope object. This object
MUST have exactly one "jmsg" field for the actual payload, "jid"
field for the identity, and "jsig" field for the signature. The
fields MUST also appear in this order. The messages MAY carry an
additional "jts" field for the timestamp, and "jsq" field for the
sequence number. If these fields are included, they MUST appear
after the mandatory fields and in the given order.
For instance, the following example contains a JSON signature
envelope and a JSON payload from a temperature sensor:
{ "jmsg": { "temp": 27.5 },
"jid": { "id": "device:cgi-27611bc81020716627ff0000cfaa1234",
"ipb": "4e26b808cd05d4e26b912ae3e26b809143fe4eb4GFTR35f82" },
"jts": { "s": 1311176727, "f": 123987 },
"jsq": 23,
"jsig": "18929abqxc67juil7ff231000912927755bRRwlkadbfddceab"}
Note that signatures envelopes can be nested; a JSON signature
envelope can be placed inside another signature envelope in the
"jmsg" field and signed. This is useful to implement secure
intermediaries that want to include additional information beyond
what the device itself provided.
5.4.1. The jmsg Field
This field MUST contain the actual payload that the device wants to
send, in the usual JSON format.
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Note that the JSON envelope needs to be useful without securing
information in the rest of the CoAP message carrying it, as well as
in situations where it is retransmitted in CoAP or HTTP via an
intermediary. For this reason all the relevant information MUST be
in the payload part. This is usually the case when taking an
information centric approach as in [I-D.arkko-core-sleepy-sensors].
The jid field carries the identity of the device, and the jmsg
carries all relevant information about what the devices wants to
communicate. Consequently, the payload SHOULD be self-contained,
without reference to the source or destination IP addresses of the
CoAP message, or to the CoAP/HTTP method or URI.
5.4.2. The jid Field
This field MUST contain an identity as defined in Section 5.3.
5.4.3. The jts Field
This field MUST contain an object with two fields. The first field,
"s", indicates the number of seconds since January 1, 1970, 00:00
UTC. At least 48 bits of accuracy is required. The second field,
"f" indicate the number of 1/64K fractions of a second, with 16 bits
of accuracy.
Implementation note: This format is compatible with the usual
representation of time under UNIX, although the number of bits
available for the integer and fraction parts may vary.
5.4.4. The jsq Field
This field MUST contain an integer representing a monotonically
increasing sequence number of all messages sent by the sender. At
least 32 bits of accuracy are required.
5.4.5. The jsig Field
This field MUST contain a variable-length string containing a BASE64-
encoded PKCS#1 v1.5 signature, constructed by using the sender's
private key over the following sequence of octets:
1. The 128-bit CGI Usage Discriminator value for this specification,
0x53eb e540 4a92 5517 57b6 e398 7aaf a085. (The value has been
generated randomly by the editor of this specification.)
2. The entire JSON payload, verbatim and in text as carried in the
message, with the contents of the jsig field set to an empty
string (jsig: "").
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The signature value is computed with the RSASSA-PKCS1-v1_5 algorithm
and SHA-256 hash, as defined in [PKCS.1.1993]. Senders use their
private key associated with the claimed identity. The "jsig" field
MUST be the last one in JSON payload. The resulting PKCS#1 v1.5
signature is put in the "jsig" field.
Receivers MUST treat messages without the "jsig" field as unsecured.
A received "jsig" field MUST be checked as follows:
o The receiver MUST ignore any fields that come after the first
"jsig" field, for both verification and other processing purposes.
o There must be an associated JSON identity information, so that
both the identity and associated public key must be apparent from
the secured message, or learned from a preceding message.
o The "jsig" field MUST have correct encoding.
o The signature verification MUST show that the signature has been
calculated as specified above.
Messages that do not pass all the above tests MUST be silently
discarded if the host has been configured to accept only secured CoAP
messages. The messages MAY be accepted if the host has been
configured to accept both secured and unsecured messages but MUST be
treated as an unsecured message. The receiver MAY also otherwise
silently discard packets (e.g., as a response to an apparent CPU
exhausting DoS attack).
6. Concluding Remarks
This memo has presented a deployment model, security architecture,
and an initial sketch of protocol design to support the architecture.
To recap, the main benefits of this model are
o Minimal configuration: per device or per group registration of
identities in a server, but no configuration in every device.
o Support for deployment models that are easily implementable by
installation personnel. The necessary practices are already
employed in typical current smart object networks, even when there
is particular support for security.
o Architecture that naturally supports information-centric
networking, multicast, middleboxes, aggregation, sleeping nodes,
and other aspects that are typical for networking for smart
objects.
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Internet-Draft CoAP Security July 20117. Security Considerations
This entire memo deals with security issues. Some analysis of the
security of the mechanisms proposed in this memo is necessary,
however.
The security of the architecture rests on the choice of the number of
bits in the identifier and the used hash and signature algorithm.
With the use of 128 bits identifiers and SHA-256 and RSA, it is
expected that the security level is similar to the one in HIP, and
goes beyond the 59 bit security of CGAs.
The basic architecture concerns itself only with integrity and data
origin verification, not about confidentiality. Where
confidentiality or identity privacy is required, additional
mechanisms are needed.
Replay attacks can be prevented beyond a small time window of
acceptable clock drift, when devices employ the optional timestamp
mechanism. This rests on the assumption of secure time
synchronization or configuration in the nodes, however. Where NTP is
used, its security properties in different modes are discussed in
Section 15 of [RFC5905]. In general, no major security problems have
been experienced with NTP protocol or reference implementation
[NTP.Wikipedia], but protection against determined hostile attackers
does require authentication at NTP the layer. Alternative, simpler
approaches include relying on the accuracy of clocks set at
manufacturing time.
The optional sequence number mechanism can prevent all replay attacks
for persistent communications between two peers. Without the use of
these two mechanisms there is no support for preventing replay
attacks. This may be acceptable in some environments, but not in
all.
Any information centric communication model is resistant to attacks
against nodes only sending information, as they are not expected to
process any security-related messages. Thus, the "sleep torture
deprivation attack" described by Stajano and Anderson in
[Resurrecting-Duckling] and other denial-of-service attacks of the
same nature are not applicable in the architecture proposed in this
memo. However, by the same token nodes that receive information
become more vulnerable to denial-of-service attacks, as nonce
exchanges, puzzles and other standard protocol mechanisms are not
used to guard against the receiver having to verify a cryptographic
operation on a received packet. The authors believe that this is the
right tradeoff for sensor networking, given that server and gateway
implementations are more likely to have the necessary capabilities to
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Internet-Draft CoAP Security July 2011
deal with attacks than sensor nodes.
8. IANA Considerations
IANA should reserve the new URN type "device" (Section 5.1). A new
registry should be created to hold subtypes of this URN type, with
the initial value "cgi" defined in this memo. New values can be
created through IETF Review or IESG Approval [RFC5226].
IANA should also create a new registry for Cryptographically
Generated Identifiers, and add a new name space Extension Type
(Section 5.2) there. Policy for adding new extensions in this
registry is RFC Required or IESG Approval [RFC5226]. Initial values
for the Extension Type field are given below. Assignments consist of
a name and the value.
Extension Type 0x0000 should be marked as reserved. Section 5.2.1
allocates Extension Type 0x0001. As recommended in [RFC3692], this
document also makes the following assignments for experimental and
testing use: the value 0xFFFD, with name Exp_FFFD; the value 0xFFFE,
with name Exp_FFFE, and the value 0xFFFF, with name Exp_FFFF.
IANA should also add another new name space to the same registry, for
128-bit CGI Usage Discriminators. These values are allocated on a
First Come, First Served basis [RFC5226]. The one initial value in
the registry is given in Section 5.4.5.
9. References9.1. Normative References
[I-D.ietf-core-coap]
Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
"Constrained Application Protocol (CoAP)",
draft-ietf-core-coap-06 (work in progress), May 2011.
[I-D.jennings-senml]
Jennings, C., "Media Type for Sensor Markup Language
(SENML)", draft-jennings-senml-05 (work in progress),
March 2011.
[RFC3279] Bassham, L., Polk, W., and R. Housley, "Algorithms and
Identifiers for the Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 3279, April 2002.
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